AT KEW OBSERVATORY AND THEIR DISCUSSION. 



461 



Recovery after Pressure Cycle. 



20. As already stated, it is customary for an aneroid to read lower on completion 

 of a pressure cycle than at its commencement. Let the departure from the original 

 reading be called the deficiency, and let D< represent the deficiency when t minutes 

 have elapsed after return to the original pressure. Thus D which we may call 

 the original deficiency is the difference between the descending and ascending 

 readings, answering to the pressure of 30 inches. 



According to the first 24 experiments, D is, at least approximately, proportional 

 to the pressure range. The evidence for this conclusion is summarised in Table XV. ; 

 the figures are deduced from the mean values calculated from the several experiments 

 over the same range. 



TABLE XV. Value of D X (1800/range) in inches. 



The abnormal entry 29 for aneroid No. 3 is really due to the occurrence of 

 appreciable permanent changes of zero in the course of 2 experiments over the 

 range 30-21 inches. On one of these occasions, after a full day's rest, the aneroid 

 read no less than '08 inch lower than before the experiment. 



The irregularity in the other results is due partly to the comparative fewness of 

 the experiments, and partly to the fact that the pressure at the end of the experi- 

 ment was in reality to some extent different from that at the start. It was usual, in 

 fact, to treat the atmospheric pressure at the time being as 30 inches, unless it 

 exceeded this value by several tenths of an inch. 



21. In the 24 experiments the aneroids were read at intervals of 5, 10, 15, 20, 

 60, 120, and 1440 minutes after the return to atmospheric pressure. The similarity 

 of the phenomena for the several aneroids and the several ranges is brought out 

 more clearly by tabulating D,/D than D ( itself. This is done in Table XVI. 

 Initially, of course, i.e., when t =. 0, D ( /D is unity. 



